Conventionally, a Schottky diode includes a heavily-doped semiconductor substrate, typically made of single-crystal silicon. A second layer covers the substrate. The second layer, called the drift region, is less heavily-doped with impurities having carriers of the same conducting type as the substrate. A metal layer or a metal silicide layer forms a Schottky contact with the lightly-doped drift region and forms the diode anode.
Two opposing constraints arise when forming a unipolar component such as a Schottky diode. In particular, the components should exhibit the lowest possible on-state resistance (Ron) while having a high breakdown voltage. Minimizing the on-state resistance imposes minimizing the thickness of the less doped layer and maximizing the doping of this layer. Conversely, to obtain a high reverse breakdown voltage, the doping of the less doped layer must be minimized and its thickness must be maximized, while avoiding the creation of areas in which the equipotential surfaces are strongly bent.
Various solutions have been provided to reconcile these opposite constraints, which has led to the development of trench MOS-capacitance Schottky diode structures, which are referred to as Trench MOS Barrier Schottky (TMBS) diodes. In an example of such devices, trench regions are formed in the upper portion of a thick drift layer that is less heavily doped with impurities of the same conductivity type than the underlying substrate. The trench regions are filled with a MOS structure. An anode metal layer is evaporated to cover the entire surface and forms a Schottky contact with the underlying drift region.
When reverse biased, the insulated conductive areas cause a lateral depletion of charge into the drift region, which modifies the distribution of the equipotential surfaces in this layer. This enables increasing the drift region doping, and thus reducing the on-state resistance with no adverse effect on the reverse breakdown voltage. However, even in these devices the electric field distribution in the drift region is still far from ideal.